It is known to use an impulsive unit to power a jack hammer or rock drill. An impulsive unit is typically powered by a closed loop hydraulic system using oil or an oil/water emulsion as the working fluid. This has the disadvantage that should the hydraulic system leak, then oil or oil/water emulsion will pollute the working environment. This is particularly undesirable in confined working environments, such as in deep mines.
Local heating is a significant issue in deep mining since the earth temperatures generally increases between 1.2° C. and 2.6° C. with each 100 meters of descent. For a 4,000 or 5,000 deep mine this could result in an ambient temperature in the region of 60° C., and consequently a significant amount of cooling must be provided in order to enable men to work at these depths.
It is also a problem that jack hammers and rock drills used in deep mines at these depths must also be cooled.
It is not possible to use water as the working fluid in an impulsive unit intended for use with oil or an oil/water emulsion as the working fluid, because the viscosity of water is significantly lower than that of oil or an oil/water emulsion. A typical impulsive unit includes a cylinder and piston, which is actuated by the working fluid. The clearance between the cylinder diameter and piston diameter in an impulsive unit for use with oil or oil/water emulsion is typically between 20 and 50 micro-meters. If such a unit is used with water, rather than oil or oil/water emulsion, such clearances are too great for water pressure to be maintained within the unit. In other words, the water is forced past the piston in the cylinder, rather than actuating the piston. Furthermore, water is not a lubricant, and so even if the clearances between the parts of the impulsive unit are reduced sufficiently to seal the piston against the cylinder, then the components which move relative to one another tend to seize or weld together.
It is an object of the invention to provide an impulsive unit, which can be powered by water and which substantially reduces the above-mentioned problems.
According to a first aspect of the present invention there is provided an impulsive unit adapted to be powered by water comprising a piston slidable within a cylinder, in which at least a portion of the piston and/or cylinder is coated with ceramic.
Preferably, the piston carries a striker portion thereon.
It has been found that by using a ceramic coating on at least a portion of the piston and/or cylinder, the clearances between the piston and the cylinder can be reduced sufficiently to enable water to be used as the working fluid, without the components seizing together. This surprising effect is thought to be caused by water particles becoming trapped in the surface pore structure of the ceramic, so that the piston and cylinder are kept spaced apart in operation. Thus the water is able to lubricate the surfaces as effectively as a film of conventional oil.
Preferably the ceramic coating contains a metal oxide. Most preferably the ceramic coating is an aluminium oxide-titanium dioxide composite.
Preferably the piston and cylinder are substantially cylindrical.
The internal wall of the cylinder may be formed by a sleeve. The clearance between the internal diameter of the cylinder and the external diameter of the piston is preferably between 8 and 10 micro-meters.
Preferably a pressure unit is provided for building up a charge of water under pressure and directing the charge of water under pressure towards a face of the piston. The pressure unit is preferably charged with between 5 and 10 cc gas at between 60 and 70 bar.
A valve means may be provided for controlling the charge of water to and from the cylinder. Preferably the valve means is a spool valve which is slidable with respect to the cylinder.
The impulsive unit may be adapted to operate when supplied with pressurized water at a rate of between 20 and 30 liters per minute. Preferably the water is pressurized to between 100 and 110 bar.
The impulsive unit may deliver around 1350 blows per minute to the tool when the water flow rate is 20 liters per minute. Alternatively the impulsive unit may deliver around 1900 blows per minute to the tool when the water flow rate is 30 liters per minute.
Preferably water is supplied to the impulsive unit from a closed loop supply, which may be cooled and/or filtered with a 10 micro-meter filter.
It is an advantage of the invention that only water is spilt in the event of a leak, which is non-toxic and not flammable. It is a further advantage of the invention that corrosion of the impulse unit is inhibited because it forms a closed loop circuit with substantially no air in the circuit.
The impulsive unit may be used in a jack hammer, a rock drill or any other impact tool.
According to a second aspect of the invention there is provided a tool system for use in mines comprising at least one water powered impulsive unit according to the first aspect of the invention.
Preferably one or more tools are mounted onto a moveable platform such that the tools may be easily advanced along a working seam. Advantageously the platform includes stabiliser means, such as one or more telescoping elements moveable to engage with the roof, sides or floor of a tunnel to hold the platform against motion or toppling when the impulsive units are in use.
According to a third aspect of the invention, there is provided a closed loop hydraulic system, in which water is the working liquid.
Preferably, the closed loop hydraulic system includes a reservoir of water, a pump for supplying water under pressure from the reservoir to a tool, and means for conveying water from the tool back to the reservoir.
The reservoir may be at atmospheric pressure, ie vented to atmosphere. Alternatively, the reservoir may be sealed to atmosphere and maintained at a pressure above or below atmospheric pressure.
Preferably, the tool is an impulsive unit. The impulsive unit may be a jack hammer or rock drill.
For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:—